British Patent Application No. 2,433,360 discloses an electronic commutator circuit for use with a stator winding of an electrical machine. The stator winding includes a number of coils linked by the same number of points of common coupling. The electronic commutator circuit includes the same number of switching stages with each switching stage being connected between a respective one of the points of common coupling and first and second dc lines. Each switching stage includes a first reverse blocking semiconductor power device capable of being turned on and off by gate control having its anode connected to the first dc line and a second reverse blocking semiconductor power device capable of being turned on and off by gate control having its cathode connected to the second dc line.
A preferred type of semiconductor power device is the Reverse Blocking Gate Turn Off Thyristor (RB-GTO) or RB-GTO derivative because they benefit from reduced on-state losses relative to other gate commutated device families such as Insulated Gate Bipolar Transistors (IGBTs) as a result of their internal regenerative properties.
The reverse blocking semiconductor power devices are used in place of conventional thyristors and this means that the electronic commutation circuit can force the commutation of current irrespective of the voltage in the stator winding.
The electric commutator circuit may also include a switching aid network (snubber) connected in parallel with each of the semiconductor power devices. The snubber may be a series connected resistor-capacitor or configured according to the well known industry standard “RCD Polarized Snubber” as shown in the figures of British Patent Application No. 2,433,360. The purpose of either type of snubber is to assist the forced commutation process and reducing switching losses in the power semiconductor devices. However, the dissipation of energy within the snubber when the outgoing RB-GTO is switched during a forced commutation process represents an appreciable energy loss and reduces the overall efficiency of the electronic commutator circuit.